1. Field of the Invention
The invention relates to a memory cell structure and method for forming the same, and more particularly, to an asymmetric memory cell structure and method for forming the same.
2. Description of the Prior Art
Semiconductor memory devices are prevalently used in computer and electronics industries as a means for retaining digital information. Typically, the semiconductor memory devices are divided into volatile and non-volatile memory devices depending on whether the data stored in the memory devices is completely lost or not in case of power interruption. And the non-volatile memory devices, which can retain their data even when the power supply is interrupted, have been widely employed.
In the conventional non-volatile memory technology, a silicon-oxide-nitride-oxide-semiconductor (hereinafter abbreviated as SONOS) memory structure is to build a silicon nitride layer sandwiched between two silicon oxide layers for serving as the charge trap layer while the two silicon oxide layers respectively serve as a charge tunnel layer and a charge block layer. This oxide-nitride-oxide (ONO) multilayered structure is further formed between a semiconductor substrate and a silicon floating gate, and thus a SONOS memory structure is constructed.
In operation, a high electric field is induced thus electrons or holes are accelerated and injected into the oxide charge tunnel layer from the source. The nitride charge trap layer traps the electrons or holes that penetrate through the charge tunnel layer. And the oxide charge block layer prevents the electrons or holes that escape the charge trap layer from reaching the silicon during program or erase operations.
When erasing information recorded in the SONOS memory structure, an electric field, in an opposite direction to that used when programming, is formed by applying a predetermined voltage to the gate after the substrate is grounded. Accordingly, the electrons or holes are removed from the charge tunnel layer and back to the drain.
It is therefore concluded that the electric fields between the substrate and the SONOS memory structures play important roles during programming and erasing the information.